The long-term objective of this research program is to develop a better understanding of (a) the photochemistry and photobiology of vitamin D3 that occurs in the skin when humans are exposed to sunlight and (b) what physiologic functions the active from of vitamin D3 (1,25-(OH)2-D3) has on human skin. During the past three years, we have developed new technology that permits us to (a) evaluate how seasonal changes affect the cutaneous production and regulation of previtamin D3 synthesis, (b) directly investigate the photodegradation of vitamin D3 using a high performance liquid chromatograph that is attached to an inline tritium detector (c) culture human keratinocytes from normal subjects and patients with dermatologic disorders such as psoriasis in a serum-free condition in order to evaluate the morphologic and biochemical responsiveness of these cells to 1,25-(OH)2-D3, and (d) evaluate the clinical effectiveness of orally or topically administered 1,25-(OH)2-D3 for the treatment of psoriasis. During the next grant period we plan to use these techniques to continue our productive research program aNd investigate in detail (1) how seasonal changes in Boston and other cities at different latitudes around the world affect the cutaneous production of preD3 and the photodegradation of preD3 and vitamin D3, (2) whether the photoproducts of preD3 have any biologic effect either on the skin or on calcium metabolism, (3) the metabolism of some of the major photoproducts of preD3 and vitamin D3 (including lumisterol, suprasterol 1, suprasterol 2, 5,6-trans D3) that occur when human skin is exposed to sunlight, (4) the biochemical functions of 1,25-(OH)2-D3 on cultured human keratinocytes with special emphasis on alterations in intracellular calcium concentrations, phosphatidylinositide metabolism and transglutaminase activity, and (5) the responsiveness of cultured keratinocytes and fibroblasts to synthetic analogs of 1,25-(OH)2- D3. Results from these investigations should provide basic new insights into (a) the role of sunlight in providing adequate vitamin-D nutrition for humans and (b) the physiologic effects of 1,25-(OH)2-D3 and its analogs on healthy human skin and potential pharmacologic effects that could be potentially used for the treatment of dermatologic disorders.